The present invention relates to an optical multiplexer and/or an optical demultiplexer which can be utilized for spectroscopic analysis in optical equipment, and/or combining and/or separation of optical signals in wavelength-division multiplexing transmission system.
A telcommunication system utilizing an optical fiber transmission system has the advantages over prior metallic cables, such as small loss, wide bandwidth, small cable diameter, light-weight cable, high-flexibility cable, no cross talk characteristic, and immunity to electromagnetic interference. And the latest developments for manufacturing optical fiber with low loss, and for controlling the wavelength in light sources, makes possible a wavelength-division multiplexing transmission system which transmits a plurality of wavelengths in a single optical fiber. This technology not only increases the transmission capacity of an optical fiber, but also makes possible a two-way transmission system and/or simultaneous transmission of a plurality of different signals.
FIG. 1 shows an example of the basic configuration of a wavelength-division multiplexing transmission system (called W.D.M.). In this figure, the signals of a plurality of channels (CH.sub.1, CH.sub.2, . . . , CH.sub.n) at the transmission side are converted to a driving signals for driving light sources by respective transmitter circuits (TR.sub.1, TR.sub.2, . . . , TR.sub.n), and are applied to light sources (T.sub.1, T.sub.2, . . . , T.sub.n). Each light source generates a light beam according to said driving signals and each of said light beams has the center wavelength (.lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n), and each of said center wavelengths corresponds to a different channel. As a light source a laster or a light-emitting-diode (LED) can be generally utilized. The output light beams from the light sources are applied to an optical multiplexer (MUL) through an optical fiber and are multiplexed or combined therein, then the multiplexed light beams are applied to an optical fiber transmission line (LINE).
At the reception side, each wavelength is separated from the others by an optical demultiplexer (DE-MUL), and each separated light beam is applied to each light detector or sensor (R.sub.1, R.sub.2, . . . , R.sub.n), which converts the optical signal to an electrical signal. Said electrical signal is applied to a corresponding output terminal through a receiver circuit (REC.sub.1, REC.sub.2, . . . , REC.sub.n).
It should be appreciated that, in a two-way WDM transmission system, both the light sources and the light detectors are provided at both the transmission side and the reception side.
The present invention provides an optical multiplexer and/or an optical demultiplexer utilized as mentioned above as an example. It should be noted that due to the reversibility of a light beam, the structure of an optical multiplexer is the same as the structure of an optical demultiplexer. Accordingly, it should be noted that the word "multiplexer" or "demultiplexer" involves both a multiplexer and a demultiplexer unless a specific definition is given.
Some of the prior devices which can be utilized as an optical multiplexer are a prism, an optical grating, and a wavelength-selective filter. A prism and an optical grating are wavelength selective devices which utilize the relationship between the wavelength and the refractive index, or diffraction angle of a prism or an optical grating, respectively.
A wavelength selective filter reflects a specific wavelength and transmits other wavelengths, and is embodied by plastics with coloring matter or dye, or a laminated thin film interference filter in which thin film multi-layers are attached on a glass substrate through vacuum evaporation.
A graded index rod lens has a radial index profile of the refractive index as shown below. EQU N(r)=N.sub.0 (1-(A.sup.2 /2)r.sup.2)
where N(r) is the refractive index at the point of the radius (r), N.sub.0 is the center refractive index, A is a constant, and r is the length from the center. When a light beam is applied to the center of the rod in the axial direction, the diameter of the beam changes periodically, and when a light beam is applied to a portion other than the center of the rod, the light beam goes in a zigzag fashion. The combination of the above characteristics of a rod lens and an interference thin film filter provides an optical multiplexer.
This optical multiplexer has a structure as shown in FIG. 2, in which an interference filter 3 is sandwiched between a pair of graded index rod lenses 1 and 2. The light beam coming into the graded index rod lens proceeds in a zigzag fashion through the graded index rod lens as shown by the arrow in the figure. When used as an optical multiplexer, the lengths of the graded index rod lenses 1 and 2 are designed to be about 1/4 of the zigzag pitch of the light beam. The interference thin film filter 3 is a reflection film made of dielectric multi-layer film with a wavelength dependency characteristic, that is, reflectivity and transmissivity of this film differs depending on the wavelength of the light
The following is a description of signals with two different wavelengths .lambda..sub.1, and .lambda..sub.2, introduced into the optical fiber 100, and separated into two different positions. The optical signal waves of two different wavelengths emitted from the optical fiber 100 proceed zigzag and propagate through the graded index rod lens 1 and enter the interference filter 3. Then, the interference filter 3 reflects the optical signal wave with wavelength .lambda..sub.1 but transmits the optical signal wave with wavelength .lambda..sub.2. The optical signal wave with wavelength .lambda..sub.1 is reflected and enters the optical fiber 101. The optical signal wave with wavelength .lambda..sub.2 propagates through the graded index rod lens 2 and is introduced into the optical fiber 102. Therefore, the two optical signal waves with different wavelengths can be separated. The characteristics of the interference filter 3 as a multiplexer are determined by the position 5 of the fiber 100. The positions 6 and 7 of the optical fibers 101 and 102 which receive the separated signal waves are also determined by the position 5 of the optical fiber 100. Therefore, the optical multiplexer as shown in FIG. 2 has the disadvantage that the characteristics of the interference filter 3 and the positions of the fibers 101 and 102 for reception of the waves cannot be adjusted independently.
When three or more waves are to be separated, the configuration shown in FIG. 3 is utilized. In this case, a plurality of graded index rod lenses 1, 2, 1', 2', 1", 2" are assembled. However, loss will be great if connecting positions of these graded index rod lenses are not controlled with precision. The larger the number of signal waves to be separated, the greater the adjustment difficulties.
Another prior optical multiplexer utilizing a wavelength selective mirror is disclosed in the U.S. Pat. No. 3,953,727. According to said U.S. patent, a plurality of selective mirrors oriented at 45 degrees in relation to the axis of the light beam are arranged in a cascaded configuration, and each selective mirror reflects a specific wavelength. Accordingly, when there are many wavelengths to be multiplexed or demultiplexed, a light beam must pass many selective filters, therefore, the transmission loss is great. Further, said U.S. patent has the disadvantage that when the wavelength to be separated is near to that of the other wavelength, separation is impossible since the angle of incidence is as large as 45 degrees, and the transmission and/or reflection characteristics of that filter depends upon whether the light beam is P-polarized light or S-polarized light.
Said U.S. patent also discloses a multiplexer in which a plurality of band pass filters are arranged around a glass plate with semi-reflective walls. However, this multiplexer has the disadvantage that the loss of the light beam is great since the light beam suffers from a plurality of partial reflections or partial transmission in said semi-reflective walls.